1. Field of the Invention
The present invention relates to the carbonylation of methyl acetate in a virtually anhydrous medium, and, more especially, relates to the carbonylation of methyl acetate in a virtually anhydrous medium, in the presence of a cobalt-based catalyst.
2. Description of the Prior Art
U.S. Pat. No. 2,730,546 describes the carbonylation of methyl acetate to prepare acetic anhydride, in the presence of a catalyst selected from among the cobalt complexes of the general formula: EQU [R.sub.4 A].sub.2 CoX.sub.4
in which X represents a bromine or iodine atom, A represents a nitrogen or phosphorous atom and R represents a lower alkyl radical, for example.
These complexes can be formed in situ by introduction into suitable reaction vessel of, firstly, a cobalt halide (CoX'.sub.2) and secondly a quaternary ammonium (or phosphonium) halide (R.sub.4 AX). The formation of the subject complexes can thus be represented by the following reaction: EQU 2(R.sub.4 AX)+CoX'.sub.2 .fwdarw.[R.sub.4 A].sub.2 CoX'.sub.2 X.sub.2.
However, the efficacy of these cobalt-based catalysts appeared to be relatively low. This type of process, the value of which is not contested in principle, has to date proven unacceptable on an industrial scale.
French Pat. No. 2,242,362 (corresponding to U.S. priority applications, Ser. No. 394,220 and Ser. No. 467,997, respectively filed on Sept. 4, 1973 and May 8, 1974the latter having issued as U.S. Pat. No. 3,904,134) describes a two-stage process for the preparation of acetic anhydride, in which, in a first step, methyl bromide or, preferably, iodide is carbonylated to provide the corresponding acetyl halide, such acetyl halide in turn being reacted with methyl acetate, in a second step, to provide acetic anhydride, which corresponds to the following reaction scheme, in the event that methyl iodide is the starting material:
Step 1: EQU CH.sub.3 I+CO.fwdarw.CH.sub.3 COI
Step 2: EQU CH.sub.3 COI+CH.sub.3 COOCH.sub.3 .fwdarw.(CH.sub.3 CO).sub.2 O+CH.sub.3 I.
As is readily apparent from this scheme, the methyl iodide, which is the starting material of step 1, is "regenerated" in step 2. Step 1 is advantageously carried out in the presence of a rhodium-based catalyst; step 2 would be assisted by the presence of lithium and/or chromium. Both steps would be assisted by the presence of lithium and/or chromium. Both steps can be carried out in one and the same reaction zone, which will then contain methyl iodide, methyl acetate, rhodium and, if appropriate, lithium and/or chromium, and even acetyl iodide, into which zone carbon monoxide will also be introduced.
U.S. Pat. No. 4,115,444 proposes an improvement to the technique described in the abovementioned French patent, which improvement consists in adding, to the reaction medium, an organic phosphorus or nitrogen compound in which the phosphorus or the nitrogen is trivalent, and confirms the potential value, in this reaction, of catalyst systems based on rhodium, or even palladium or iridium, and chromium.
French Pat. No. 2,303,788 (corresponding to U.S. priority applications Ser. No. 556,749 and Ser. No. 654,662, respectively filed on Mar. 10, 1975 and Feb. 5, 1976) reflects that the presence of a large amount of hydrogen in the reaction medium above described has a considerable influence upon the direction of the reaction. In fact, under these conditions, a mixture is obtained which contains a preponderant proportion of acetic acid and variable amounts of ethylidene diacetate, acetic anhydride and acetaldehyde.
The principal value of these processes employing catalysts based on rhodium, or even palladium or iridium, the systems based on the pair (rhodium/chromium) appearing to be the most active, essentially resides in the possibility which they present of obtaining acetic anhydride starting from methyl acetate, utilizing carbon monoxide partial pressures which are lower than those required in the earlier processes.
Nevertheless, the attempts to develop this type of process, even on a simple pilot plant scale, have encountered serious difficulties.
A first series of difficulties arises from the fact that the catalysts based on rhodium or palladium, or even iridium, which metals are extremely rare and expensive, are deactivated, in particular during the treatments required to recover the reaction product (or products). Because of the cost of these catalysts, it is essential to regenerate same. Furthermore, the conditions required to convert these metal compounds to catalytically active species in the carbonylation reaction are most frequently incompatible with those required to maintain the chromium-based co-catalysts in their active form in this same reaction. Still further, the losses of rhodium, for example, which seem to be unavoidable at the various points in an industrial plant, severely impair the economics of such a process.
A second series of difficulties is derived from the presence, required for the reaction to proceed well and for the stabilization of the rhodium, of large amounts of methyl (or acetyl) iodide, which involves significant risks of corrosion at the various points in an industrial installation. Furthermore, the methyl iodide and/or certain of its derivatives formed in the reaction medium are responsible for an unacceptable contamination of the reaction product (or products), which makes it necessary to carry out additional steps in order to remove the iodides whose presence in the reaction products proves to be undesirable. For obvious economic reasons, these iodine derivatives, which are present in large amounts not only in the products but also in various effluents originating from the reaction zone, must be recovered, and this involves additional treatment stages.
The various problems associated with this type of process, which are difficult to solve, will become more clearly apparent from French Pat. Nos. 2,438,023 and No. 2,438,024 (corresponding respectively to U.S. priority applications, Ser. No. 949,344 and Ser. No. 949,345, filed Oct. 6, 1978these applications having issued as U.S. Pat. Nos. 4,252,741 and 4,246,192, respectively) and U.S. Pat. No. 4,241,219.
Too, it is also well known that methyl acetate can be obtained by reacting acetic acid with methanol, it being possible for the acetic acid to be produced by the carbonylation of methanol and for the methanol in turn to be prepared by the hydrogenation of carbon monoxide. The reactions in question can be represented as follows: EQU CO+2H.sub.2 .fwdarw.CH.sub.3 OH (a) EQU CH.sub.3 OH+CO.fwdarw.CH.sub.3 COOH (b) EQU CH.sub.3 COOH+CH.sub.3 OH.fwdarw.CH.sub.3 COOCH.sub.3 +H.sub.2 O (c)
The carbonylation of methyl acetate in a substantially anhydrous medium makes it possible to obtain acetic anhydride according to the following reaction: EQU CH.sub.3 COOCH.sub.3 +CO.fwdarw.CH.sub.3 CO--O--COCH.sub.3 (1)
Thus, the value of a process for the carbonylation of methyl acetate to yield acetic anhydride (1) is clearly apparent if reactions (a) to (c) above are also considered, since, overall, this sequence amounts to a process by which acetic anhydride is produced beginning from carbon monoxide and hydrogen.
Furthermore, cobalt being a common metal, its use in a process for the carbonylation of methyl acetate would be desirable. Nonetheless, to date the prior art has almost exclusively focused upon the rhodium based catalyst systems in such process.